This invention relates to the field of integrated circuit fabrication. More particularly, this invention relates to integrated circuit packaging materials.
Integrated circuits are preferably packaged prior to use, to protect the integrated circuit and to more easily provide for electrical connections between the integrated circuit and the external portions of the circuit in which the integrated circuit is to be used. While the packaging materials provide these valuable function, they also tend to create other problems, which should be identified and overcome to enhance the longevity of the integrated circuit.
For example, many packaging materials tend to have relatively low thermal conductivities. When the integrated circuit is operating, thermal energy is developed by electron motion in different parts of the monolithic integrated circuit. Because the packaging materials tend to be inefficient in dissipating the thermal energy, it tends to be expressed as an increase in temperature in various parts of the integrated circuit, which localized temperature increases are typically referred to as hot spots.
Over time, the thermal energy may conduct throughout the bulk substrate material of the monolithic integrated circuit, which may then tend to be at a relatively uniform temperature. The monolithic integrated circuit thus tends to swell and contract, based at least in part upon the temperature of the substrate. The degree to which the integrated circuit swells and contracts is predominantly dependent upon the temperature coefficient of expansion of the substrate. However, the packaging materials surrounding the monolithic integrated circuit tend to have temperature coefficients of expansion that are different from that of the integrated circuit substrate. Thus, and in part because they tend to not conduct heat at the same rate as the integrated circuit substrate, the packaging materials, such as the underfill material, package substrate, and encapsulants, tend to expand and contract at a different rate and to a different degree than the integrated circuit.
With the various elements of the packaged integrated circuit expanding and contracting in different ways, as described above, stresses are set up in the packaged device. These stresses are often strong enough to twist or otherwise warp the package. As the package twists and bends to alleviate the stresses, mechanical failures may occur in the package, such as electrical connections breaking. Obviously, such conditions reduce the utile life of the packaged integrated circuit.
One method of reducing the twisting and warping of the package is to include a member inside the package that can stiffen the package to some degree. However, secondary circuit structures, other than the integrated circuit, that are mounted on the package substrate and included within the packaged integrated circuit tend to get in the way of such stiffeners and reduce their effectiveness.
What is needed, therefore, is an integrated circuit package design that can help reduce the twisting and warping of package materials due to thermal expansion and other physical stresses, while accommodating secondary structures such as on substrate capacitors and resistors.
The above and other needs are met by a stiffener for reinforcing a package integrated circuit. The stiffener includes a rigid planar element having a first surface for bonding to a package substrate. The rigid planar element forms a major interior aperture for receiving and surrounding an integrated circuit on all sides of the integrated circuit. The rigid planar element also forms a minor interior aperture for receiving and surrounding a secondary circuit structure on at least three sides of the secondary circuit structure.
In this manner, the stiffener provides structural support to the integrated circuit package, which reduces and preferably eliminates twisting and warping of the substrate package as it heats and is subjected to other stresses. Because the major interior aperture does not need to be large enough to fit both the monolithic integrated circuit and the secondary circuit structure, there is more stiffener material available to provide structural support than there would be if the major interior aperture was large enough to fit both the monolithic integrated circuit and the secondary circuit structure.
In various preferred embodiments, the rigid planar element forms a plurality of minor interior apertures for receiving and surrounding a plurality of secondary circuit structures on at least three sides of each of the secondary circuit structures. The minor interior aperture formed by the rigid planar element most preferably surrounds the secondary circuit structure on all sides of the secondary circuit structure. Preferably, the stiffener is formed of metal, and most preferably is formed of copper. The stiffener is preferably about fifty millimeters square in size and about six hundred and forty-five microns in thickness, and most preferably substantially the same size as the laminate package substrate, with a thickness that is lower than the monolithic integrated circuit substrate attached to the laminate package substrate. Such a size tends to minimize the overall impact on the completed package. The major interior aperture is preferably about twenty-two millimeters or about twenty-four millimeters square in size, and the minor interior aperture is preferably about four millimeters square in size. In one embodiment the major interior aperture has beveled corners. In one embodiment the major interior aperture adjoins the minor interior aperture.
According to another aspect of the invention there is described a stiffener for reinforcing a packaged integrated circuit, where the stiffener has a rigid planar element having a first surface for bonding to a package substrate. The rigid planar element forms a major interior aperture for receiving and surrounding an integrated circuit on all sides of the integrated circuit, where the major interior aperture has beveled corners.
In yet another aspect of the invention there is described a packaged integrated circuit having a package substrate, with a monolithic integrated circuit and a secondary circuit structure mounted to the package substrate. A stiffener is mounted to the package substrate for reinforcing the packaged integrated circuit. The stiffener forms a major interior aperture for receiving and surrounding the integrated circuit on all sides of the integrated circuit, and also forms a minor interior aperture for receiving and surrounding the secondary circuit structure on at least three sides of the secondary circuit structure. A cover encloses the integrated circuit and the secondary circuit.